The present invention relates to a datum beam projecting apparatus for use with surveying equipment such as an optical leveling type to provide a datum level in construction surveying or the like.
Although the datum beam projecting apparatus for surveying equipment should ideally be installed on a horizontal trestle, this apparatus has usually been installed, in practice, inevitably with a slight tilt and it has been required to compensate for such a tilt of the apparatus to achieve an effective projection of the datum beam. As an example of the well known apparatuses provided with means to maintain the direction in which the datum beam is projected constant by compensating for the tilt of the apparatus, Japanese Disclosure Gazette No. 1985-200117 discloses a datum beam projecting apparatus used with surveying equipment to project light rays emitted from a light source as the datum beam through a projecting lens system. This system includes a suspended reflector member of a complicated structure, for example, an erect prism or a Porroprism. Another example is disclosed in Japanese Disclosure Gazette (of Utility Model Application) No. 1986-105811, which includes a complicated structure comprising a pair of two-face reflectors individually suspended so that respective optical paths thereof extend perpendicularly to each other. Further, another example is disclosed in Japanese Disclosure Gazette No. 1988-179208, which includes a projecting lens system comprising a convex lens fixed to the apparatus and a concave lens suspended in an intricated distance-relationship with said convex lens. However, all of such well known apparatuses are inconveniently intricate in their structures which, in turn, increase factors not only making the accuracy unreliable but also making the cost of manufacturing unacceptably high.